A switch-mode power supply is an electronic power supply that uses a switching regulator to generate the supply voltage. A high switching frequency may cause an increase of the so-called driving losses and of the so-called switching losses. While driving losses are caused by the electrical power required for switching the switching elements, there are two different types of switching losses. A first type of switching losses is given by the simultaneous presence of current through the switching elements and voltage across their terminals during the switching thereof (“hard switching” condition). The second type of switching losses, typically called capacitive losses, is caused by the parasitic capacitance associated with each switching element, which is discharged on the resistance of the switching element itself while this is activated. Both capacitive and switching losses are proportional to the operating frequency of the switching elements. Capacitive losses are proportional to the squared switched voltage.
In order to reduce switching losses and allow high frequency operation, resonant conversion techniques have been widely developed. These techniques provide for processing electrical power in a sinusoidal manner, and controlling the switching elements in such a way to limit the occurrence of hard switching.
Resonant converters operated from the rectified power grid voltage are typically realized using the half-bridge and the full-bridge topologies. With reference to resonant DC-DC (direct current to direct current) converters implemented according to the so-called half bridge topology (briefly referred to as half-bridge resonant converters), the switching elements include a high-side transistor and a low-side transistor connected in series between the supply circuit providing the supply voltage to be converted and a terminal providing a reference voltage, such as ground. By properly switching said two transistors it is possible to generate a square wave having a high value (assumed when the high-side transistor is activated) corresponding to the supply voltage and a low value (assumed when the low-side transistor is activated) corresponding to the ground. A small dead-time, where both transistors are off, is typically inserted as soon as each transistor turns off.
The same square wave may be generated by using two pairs of switching elements each one arranged according to the half-bridge topology, but driven in phase opposition to each other. Particularly, the high-side transistor of the first pair and the low-side transistor of the second pair are activated simultaneously; similarly, the low-side transistor of the first pair and the high-side transistor of the second pair are activated simultaneously. This topology is generally referred to as full-bridge topology. A resonant converter based on a full-bridge topology is briefly referred to as a full-bridge resonant converter.
In a resonant converter, the square wave, generated with either a half-bridge or a full-bridge topology, is applied to the primary winding of a transformer through a resonant network that includes at least a capacitor and an inductor; the secondary winding of said transformer feeds a rectifier circuit and a filter circuit for providing an output DC voltage. The value of the output DC voltage depends on the frequency of the square wave, whether it gets closer to or further from the resonance frequency of the resonant network. The duty cycle of the square wave is typically kept at about 50%.
Among the various known configurations of resonant network in resonant converters, the so-called inductor-inductor-capacitor (LLC) configuration is especially suited for those applications in which the value of the DC voltage to be converted is particularly high, such as the one generated through the rectification of the power grid voltage, i.e., in a condition favorable for the occurrence of high capacitive losses. The resonant network of an LLC resonant converter is formed by a series inductor-capacitor (LC) circuit connected between the switching elements and an input of the primary winding of the transformer, and a shunt inductor connected across both the inputs of the primary winding.
With an LLC resonant converter, it is possible to adjust the value of the output DC voltage over wide load and input DC voltage variations with a relatively small variation of the switching frequency.